The invention relates to an operating circuit for a discharge lamp with preheatable electrodes.
It is generally known that it is necessary in the case of low-pressure discharge lamps for the electrodes, generally incandescent filaments, to be preheated before igniting the discharge. Proper preheating not only eases the starting operation, but is essential, in particular, for the service life of the electrodes.
It is conventional to make use for this purpose of a circuit which is connected in parallel with the discharge path through the discharge lamp and is connected in series with the two electrodes of conventional discharge lamps and includes a parallel circuit composed of a capacitor and a PTC thermistor (PTC element). During a cold start, the PTC thermistor is conducting, and so the capacitor is bridged. The operating circuit applies an operating voltage to the discharge lamp but, owing to the PTC thermistor, said voltage leads to a relatively high current which heats up electrodes or incandescent filaments through which this current flows. After a specific preheating time, the PTC thermistor becomes highly resistive, and so only the reactance of the capacitor is now active.
Consequently, the voltage between the electrodes rises, and so the discharge can be ignited.
This solution is associated with various disadvantages. Firstly, in continuous operation the PTC thermistor results in a power consumption responsible for somewhat impairing the efficiency. Moreover, thermal problems can follow therefrom for ballasts. Secondly, the PTC thermistor requires a comparatively long time, for example two minutes, for cooling after the discharge lamp has been switched off. Thus, when the discharge lamp is switched on again later after being switched off (possibly inadvertently), this restarting is performed without proper preheating of the electrodes. Finally, the capacitor used must be designed for the voltages present at the discharge lamp during ignition, and is therefore a comparatively expensive component. Again, the PTC thermistor is a component which overall increases noticeably the costs of the operating circuit.
The aim is therefore to find alternative possibilities for preheating the electrodes in the discharge lamps.
Such a possibility is outlined in U.S. Pat. No. 5,831,396. It is proposed there for an operating circuit which has a half-bridge oscillator with two bipolar transistors as switching transistors to vary the emitter resistances of these bipolar transistors upon expiry of the preheating time, it thereby being possible to influence the negative feedback in the half-bridge oscillator. It is a technical requirement of the circuit represented there to use a toroidal-core transformer which is saturated in normal operation at different instants as a function of the emitter resistances. Consequently, the variation in the negative feedback influences the operating frequency of the operating circuit. In this case, the operating circuit is designed such that during the preheating it is at an excessively high level with reference to a resonant frequency given by a lamp circuit, and it is not brought to a value which leads to ignition of the discharge lamp until after the preheating time has elapsed in the way described.
The technical problem on which the invention is based is to specify a novel operating circuit for discharge lamps with preheatable electrodes which permits a solution of the preheating of the electrodes which is functionally reliable, flexible and cost-effective.
Provided for this purpose in accordance with the invention is an operating circuit for a discharge lamp with preheatable electrodes, which operating circuit has an oscillator circuit with at least one switching transistor for generating an output power at an RF frequency for the discharge lamp, to be connected to the oscillator circuit, by means of a switching operation corresponding to the RF frequency, the operating circuit being designed such that after the operating circuit has been started the RF frequency of the oscillator circuit is varied such that the discharge lamp does not initially ignite, but a preheating current for preheating the electrodes flows through the electrodes, and after a preheating time, the RF frequency being returned to an operating frequency in the vicinity of a resonant frequency of the oscillator circuit in order to ignite the discharge lamp, characterized in that the RF frequency of the oscillator circuit is determined by at least one dedicated resonant circuit which is connected to a control electrode of the switching transistor, in order to apply control signals at the RF frequency determined by the resonant circuit to the control electrode, the variation in the RF frequency for preheating purposes being performed by a detuning of the natural frequency of the resonant circuit with reference to the resonant frequency of the oscillator circuit.
Preferred refinements of the invention are specified in the dependent claims.
Thus, an independent resonant circuit is used in the invention in order to fix the operating frequency. This resonant circuit is connected to the control electrode of the at least one switching transistor of the oscillator circuit of the operating circuit, and therefore impresses the resonant circuit frequency on the switching operation of the switching transistor, and thus on the overall operating circuit. When more than one switching transistor is provided, it is possible, furthermore, to provide two or more resonant circuits.
In this case, the statement that the resonant circuit or circuits fix the operating frequency must not be understood to the effect that it would thereby be possible to select any desired resonant frequency of a resonant circuit as operating frequency. Since the switching transistors belong to the oscillator circuit, and the latter has a specific resonant frequency, it is possible overall to have as operating frequency only frequencies in a certain environment around this resonant frequency. Thus, if the frequency of the resonant circuit should be very strongly detuned with reference to the resonant frequency of the oscillator circuit, no operation occurs. However, the operation is determined by the frequency of the resonant circuit within a specific environment around the resonant frequency of the oscillator circuit. The resonant circuit and the oscillator circuit are coupled in this case by the driving of the switching transistors, at least. However, it is also preferably provided to feed back energy from the oscillator circuit into the resonant circuit in order to couple energy into the resonant circuit.
The resonant circuit according to the invention is independent with reference to the oscillator circuit to the extent that it fixes an independent frequency and can be tuned independently in frequency and thus, in particular, also be changed, that is to say detuned. The invention now provides to vary the frequency-determining variables directly in the resonant circuit according to the invention, instead of, as proposed in the quoted prior art, influencing the circuitry of the switching transistors, and thus influencing the feedback of the oscillator circuit. The resonant circuit is therefore to be specifically detuned in order to permit the preheating operation. Thus, during the preheating time it is frequency-shifted with reference to the or those operating frequencies which result in ignition of the discharge lamp, and is not varied so that the discharge lamp can ignite until expiry of the preheating time.
It is thereby possible to implement particularly simple and efficient circuits; in particular, there is no need to use the PCT thermistor and the capacitor conventionally connected in parallel therewith.
Moreover, the concept according to the invention is capable of relatively universal use, because the selection of transistors as switching elements in oscillator circuits can be effected in various way, not only in the form of bipolar transistors. In principle, there are no other essential preconditions than the use of at least one switching transistor in the oscillator circuit. The use of a toroidal-core transformer is also not necessary. Rather, a transistor which does not saturate in normal operation is preferred for feedback into the resonant circuit.
A half-bridge arrangement which includes two switching transistors is preferred as oscillator circuit. In this case, a dedicated resonant circuit can be provided per switching transistor. However, it is also possible to find solutions which manage with a single resonant circuit. In particular, the signal of a resonant circuit could be inverted for one of the two switching transistors, while it is applied unchanged at the other one. However, it is preferred to use complementary switching transistors in the half bridge, that is to say to use a pair composed of an npn and a pnp switching transistor or a pair composed of an n-channel and a p-channel FET. The switching elements preferred according to the invention are voltage-controlled, that is to say FETs or IGBTs, in particular MOSFETs.
If a bridge circuit (in the general sense, that is to say including a full bridge) is provided, at least one resonant circuit is preferably provided between the control electrode of the respective switching transistor and the bridge midpoint, as the exemplary embodiment also shows.
The resonant circuit is preferably a resonant circuit in which the frequency is defined by one or more inductances and one or more capacitances, that is to say an LC resonant circuit. As the exemplary embodiment makes plain, the capacitance can also be a transistor input capacitance. A dedicated capacitor is not mandatory.
The detuning of the resonant circuit can be performed in different ways, for example by varying the effective frequency-determining inductance or capacitance. However, it is preferred to switch a capacitance in or out. A switch, in particular a transistor, is therefore provided for this purpose in a section, connected to the capacitance, in the resonant circuit.
In this case, during the preheating the frequency will preferably be higher than the continuous operation frequency, and so the resonant circuit will be detuned toward yet higher frequencies. Thus, when it acts in parallel with other capacitances the above-named capacitance can be switched out during preheating, and so the corresponding line section is interrupted. Upon termination of the preheating time, the section is then switched in, and so the capacitance also determines the frequency. The reverse applies in the case of a series interconnection with other capacitances. For example, it is possible during preheating to use a small transistor input capacitance, while the capacitance to be switched in upon termination of preheating is present in the form of a dedicated capacitor. Reference is made to the exemplary embodiment.
The preheating time can be defined in various ways. A preferred solution consists in a circuit which recharges a preheating capacitance during the preheating time, and leads to a changeover of the frequency in the resonant circuit when a specific voltage is reached across the capacitance.
The invention fundamentally offers the advantage of a substantially shorter time interval between switching off and switching on again to the accompaniment of proper preheating. In the case of the solution proposed here for defining the preheating time, this time interval can be further shortened if required by virtue of the fact that a discharging resistor is connected in parallel with the preheating capacitance. However, the discharging of the capacitor itself, which is conditioned by the components, can basically already effect sufficiently rapid discharge after the switching off.
The voltage threshold value in the case of the charging of the preheating capacitance can be defined, for example, by a zener diode. The exemplary embodiment shows how, after the conducting-state voltage of the zener diode has been overcome, a transistor is turned on which is arranged in the section situated at the capacitor for frequency detuning (toward the operating frequency). The zener diode then serves the purpose of increasing the threshold voltage prescribed by the transistor.
It is also possible to provide in parallel with the control junction of the transistor a further resistor which renders the circuit less sensitive with regard to fluctuations in the transistor, that is to say approximately in parallel with the emitter-base junction, provided here in the exemplary embodiment, of the bipolar transistor of the preheating circuit.
When the already mentioned switching transistor input capacitances are not sufficient for adequate oscillation of the oscillator circuit during preheating, the described transistor junction, which is interrupted during preheating and is thereafter to be conducting, can be connected in parallel with a relatively small capacitance in order to bring these preheating oscillations to an adequate level and thus to permit an adequate preheating current.
Of course, corresponding solutions are also possible when an inductance is used in the resonant circuit instead of the capacitor for frequency detuning.
The invention is basically directed to operating circuits for discharge lamps, in particular to operating circuits for low-pressure discharge lamps. Consequently, it is preferably applied in ballasts which can be designed separately or in an integrated fashion. A claim is therefore also being made for ballasts, configured according to the invention, for tubular fluorescent lamps which are designed separately as a rule. On the other hand, the invention is also directed to ballasts for compact fluorescent lamps, specifically both as separate ballasts and as integrated ones. In the latter case, the complete compact fluorescent lamp is therefore being claimed.